No double moon in 2018, or ever

This image sometimes circulates on Facebook, with the claim that Mars will appear as big and bright as a full moon. It’s a hoax. Don’t believe it.

Will Mars and the moon will appear the same size in 2018? No. What’s really amazing is the staying power of this hoax, which has its roots in a real 15-year cycle of Mars, that’s peaking – giving us an excellent year to observe Mars – in 2018.

We are likely to see it as an email – or on social media – in the form of a claim that Mars will appear as large as a full moon in Earth’s sky on a particular date. Sometimes there’s a suggestion that Mars and Earth’s moon will appear as a double moon. And that is just not true. It’s not true in 2018. It’s never been true. It never will be true.

Mars can never appear as large as a full moon as seen from Earth. As seen from Earth, in months when Mars does appear side by side with a full moon, Mars’ diameter appears, on average, about 1/140th the diameter of the full moon.

Mars is the planet orbiting the Sun one step outward from Earth’s orbit and is slightly smaller than Earth – but slightly larger than Earth’s moon. Mars is also much much farther away than Earth’s moon. It’s hard to comprehend what little specks the planets and moons are in contrast to the vastness of space.

Earth’s moon is about a light-second away. Light bouncing from the moon’s surface takes about a second to reach us here on Earth. Meanwhile, light from Mars takes much much longer to reach Earth – from several minutes to about 20 minutes – with the difference being the result of Earth’s and Mars’ motions around the sun. In other words, when Mars is on the same side of the sun as Earth, its distance from us is less than when it’s on the far side of the sun from us.

The moon is much closer than Mars, and that’s why we see the moon as a bright disk in our sky. Meanwhile – to the eye – Mars appears as a reddish star-like point.

So how did this rumor of Mars-as-big-and-bright-as-the-moon get started? It started in 2003. On 27th August 2003, Earth and Mars came very slightly closer than they’d been in nearly 60,000 years. Center-to-center, Earth and Mars were less than 35 million miles apart – just over three light-minutes apart. The last people to come so close to Mars were Neanderthals. Astronomy writers had a field day that year, talking about Mars at its closest.

Was it a spectacular sight? Yes! Mars looked like a dot of flame in the night sky. Was Mars as big and bright as the moon, even at its closest in 2003? Never.

The 2003 event was part of a 15-year cycle for Mars. Think of Earth and Mars in orbit around the sun again. Neither Earth nor Mars has a circular orbit. Both worlds have elliptical orbits. So both Earth and Mars have a closest point to the sun. When Earth passes between the sun and Mars (opposition) around the time Mars is closest to the sun (perihelion) – Earth and Mars come closest.

Diagram by Roy L. Bishop. Copyright Royal Astronomical Society of Canada. This diagram explains why, in 2016, Mars was closer than it had been in 10 years. In 2018, it’ll be even closer … but never moon-sized in Earth’s sky.

If we look closely at this diagram, we can see that Earth and Mars will have a particularly close opposition in mid-2018. Mars will be closer than it’s been since 2003!

It’ll be bright and very reddish but only like a dot of flame.

Source - EarthSky.org